1. Field of the Invention
The present invention relates to a grinding body for an on-line roll grinding device which is mounted and used on a rolling mill.
2. Description of the Related Art
When on-line roll grinding is performed, it is common practice, as shown in FIG. 16, that a plurality of on-line roll grinding devices 20 are placed to face a roll 21 to be ground, each of the on-line roll grinding devices 20 having a grinding body 22 capable of reciprocating in an axial direction of the roll 21 and rotatable along the axial direction, and the grinding body 22 is pressed against a surface of the roll 21, which is rotating, to grind the surface of the roll 21. A shaft center 20a of the grinding device 20 is set at the same height as an axis 21a of the roll 21, or at a height displaced upward or downward (an offset height H) by a certain distance from the axis 21a. The shaft center 20a of the grinding device 20 is also set to be horizontally inclined at an angle of .alpha. (e.g., 0.5.degree.) from a line 20b perpendicular to the axis 21a of the roll 21. This angle of inclination, .alpha., is called a grindstone pressing angle.
Such grinding of the roll 21 with the grinding device 20 is known to pose the following problems: The offset height H and the grindstone pressing angle .alpha. that have been set vary because of wear of the roll 21 by rolling, or owing to adjustment of a gap between the upper and lower rolls 21 and 21. Thus, the grindstone contacts the surface of the roll 21 unevenly, forming a spiral mark and deteriorating the roll surface. Eventually, the roll becomes unusable. Furthermore, roughening of the surface of the roll 21, and vibrations of the roll 21 due to an increased gap between the roll surface and the grinding body 22, cause the vibration of the grinding body 22, thereby forming a pitching surface mark 23 with a streaked pattern, as shown in FIG. 17, on the surface of the roll 21 to be ground. Rotary grinding bodies for preventing the formation of the pitching surface mark 23 or the spiral mark were proposed by 1 Japanese Unexamined Patent Publication No. 6-47654 (hereinafter referred to as the earlier technology I) 2 Japanese Unexamined Patent Publication No. 9-1463 (hereinafter referred to as the earlier technology II), and 3 Japanese Unexamined Utility Model Publication No. 62-95867 (hereinafter referred to as the earlier technology III).
The earlier technology I, as shown in FIGS. 18 to 19, tries to prevent the formation of the pitching surface mark 23 by securing a thin grindstone 32 onto a flexible, thin, circular base plate 31 having a central portion rotatably supported to constitute a low-rigidity grinding body 22, and absorbing vibrations of the roll 21, during grinding, by local warpage of the thin, circular base plate 31 of the grinding body 22 pressed against the roll 21. FIG. 20 shows a state in which only an outer edge of the grindstone 32 contacts the roll 21, so that the thin, circular base plate 31 warps, thus bringing the entire width of the grindstone into contact with the roll 21. FIG. 21 shows a state in which only an inner edge of the grindstone 32 contacts the roll 21.
The earlier technology II focuses on the fact that when the grinding body of the earlier technology I contacts the roll 21 at a circumferential portion of the thin grindstone 32, as shown in FIG. 21, only the warpage of the thin circular base plate 31 is not enough to resolve the uneven contact. In light of this fact, the earlier technology II, as shown in FIGS. 22 to 25, secures a cup-shaped grindstone 42 onto a circular base plate 41 having an inward groove 43 defined by a circumferential portion of the circular base plate 41 bent on a surface side, thereby constituting a grinding body. Making use of the groove 43, the earlier technology II attempts to resolve the contact of only the outer edge or the inner edge of the cup-shaped grindstone 42 with the roll surface, thereby preventing the formation of the spiral mark.
The earlier technology III, as shown in FIGS. 26 to 27, tries to prevent the formation of the pitching surface mark by fixing a cup-shaped grindstone 52 having a bottom plate to a circular base plate 51 by means of a nut 55, with the bottom plate being sandwiched between rubber plates 53 and 54 (FIG. 26), or fixing a bottom plate of a cup-shaped grindstone 52 to a circular base plate 51 by means of a nut 55, with a rubber plate 53 being sandwiched therebetween (FIG. 27), so that vibrations of the roll 21 will be absorbed by the rubber plate 53 (54).
With the grinding body of the earlier technology I, the pitching surface mark 23 has been assumed to occur because of vibrations of the roll 21 during on-line grinding. As a countermeasure, the circular base plate has been thinned to impart low rigidity to the grinding body. However, the thinning of an abrasive grain layer and a support portion (collectively called a grindstone) to impart low rigidity because of emphasis on flexibility involves the following problems:
(1) Vibrations occurring in the grinding body 22 during grinding include resonance vibrations associated with vibrations of the roll 21, and self-excited vibrations associated with stick-slips at the interface between the grindstone and the roll 21 in contact with each other. The self-excited vibrations occur because of the low dynamic stiffness of the support member for the grindstone, i.e., the circular base plate. The self-excited vibrations lead to the formation of the pitching surface mark 23.
(2) Since the support member for the grindstone is a flexible, thin, circular base plate, uneven contact of the grindstone with the roll is liable to occur, under a high grinding force, according to changes in roll setting. Thus, the oscillating speed and the grinding force are restricted, so that the grinding power declines.
(3) If the thickness of the abrasive grain layer secured to the thin circular base plate differs, the rigidity of the grindstone also varies. FIG. 13 is a graph showing the relationship between the thickness of a grindstone and the rigidity of the grindstone. As a one-dot chain line in the drawing indicates, decreases in the grindstone thickness result in rapid decreases in the grindstone rigidity. Thus, the accuracy of grinding lowers according to changes in the rigidity of the grindstone.
To retain the grindstone rigidity, the abrasive grain layer can be thickened only up to a predetermined thickness. Thus, the life of the grindstone shortens.
(4) When the grindstone supported on the flexible thin circular base plate is pressed against the roll with a predetermined pressing force, local warpage occurs, and the stress of the grindstone at the site of warpage increases. Thus, the pressing force is limited to a level at which the imposed stress is below the allowable grindstone stress. Consequently, the grinding power is restricted. FIG. 14 is a graph showing the relationship between the grindstone pressing force and the grindstone stress. As indicated by a one-dot chain line in the drawing, the imposed stress exceeds the allowable grindstone stress when the pressing force is about 50 kgf or more.
(5) To reduce the weight of the rotary movable portion, the abrasive grain layer needs to be thinned. Since the thickness of the abrasive grain layer is thus restricted, the life of the grindstone becomes short.
With the earlier technology II, special deformation of the grooved circular base plate 41 has resolved uneven contact of the grindstone with the roll. However, the pitching surface mark associated with self-excited vibrations, the problem with the earlier technology I, has not been resolved.
According to the earlier technology III, the pitching surface mark has considerably been diminished because of the effect of the rubber plate. However, its diminution has not been complete. The reason is that the inserted rubber plates 53 and 54 are exposed to the outside, so that the damping effect of rubber has not been fully exhibited owing to the penetration of foreign matter or the deterioration of rubber.